Infra-Red Spectra of Hydrocarbons: II. Analysis of Octane Mixtures by the Use of Infra-Red Spectra Obtained at Low Temperatures

Abstract

Infra‐red spectra in the region 2 to 22 microns were obtained for the four trimethylpentanes, at temperatures of 0°C and −195°C. In agreement with theory, the band ``widths'' at −195°C are roughly half as great as at 0°C. The spectra were observed experimentally by condensation of a film of the hydrocarbon on the reflecting bottom surface of a cell mounted vertically, with its lower end immersed in ice or liquid nitrogen. A method of forming films of reproducible dimensions and thickness is described. To provide a quantitative estimate of the advantage for analytical purposes of the use of spactra obtained at low temperatures, an expression is derived which gives the probability (for random distribution of the band positions) that at least one band can be found for each component of a mixture, which does not ``overlap'' bands of the other components. Application of the formula, with some simplifying assumptions, to a mixture of the 18 octane isomers, indicates that reduction of the band ``width'' from 40 cm<sup>−1</sup> to 20 cm<sup>−1</sup> (corresponding to the temperature change from 0°C to −195°C) increases the calculated probability of the existence of a ``non‐overlapped'' band for at least 17 of the isomers from 0.003 to 0.999. The importance of this result in infra‐red analyses is pointed out. The use of spectra obtained at liquid helium temperatures is suggested as offering attractive possibilities for the analysis of complex mixtures.